Sunan an-Nasa'i, Volume 2, Book 12, Hadith 1126 (DarusSalam) ⓘ

It was narrated that Aishah said: "I noticed that the Messenger of Allah was missing and I thought he had gone to visit one of his concubines, so I looked for him and found him prostrating and saying: 'Rabbighfirli ma asrartu wa ma a'lant (O Allah, forgive me for what (sin) I have concealed and what I have done openly).'"

أَخْبَرَنَا مُحَمَّدُ بْنُ الْمُثَنَّى، قَالَ حَدَّثَنَا مُحَمَّدٌ، قَالَ حَدَّثَنَا شُعْبَةُ، عَنْ مَنْصُورٍ، عَنْ هِلاَلِ بْنِ يِسَافٍ، عَنْ عَائِشَةَ، - رضى الله عنها - قَالَتْ فَقَدْتُ رَسُولَ اللَّهِ صلى الله عليه وسلم فَظَنَنْتُ أَنَّهُ أَتَى بَعْضَ جَوَارِيهِ فَطَلَبْتُهُ فَإِذَا هُوَ سَاجِدٌ يَقُولُ ‏ "‏ رَبِّ اغْفِرْ لِي مَا أَسْرَرْتُ وَمَا أَعْلَنْتُ ‏" ‏ ‏.‏